FIELD OF THE INVENTION
The invention relates to a method for triggering a DC motor through an integrated H-bridge circuit, particularly in motor vehicles, and to an apparatus for performing the method, including one high-side transistor switch between each motor terminal and a positive pole of an operating voltage, one low-side transistor switch between each motor terminal and a negative pole of the operating voltage, and one guard circuit, assigned to each motor terminal, for preventing the occurrence of harmful substrate currents.
Integrated H-bridges, which are described in more detail below with regard to FIG. 1, include four monolithically integrated transistors. Depending on the direction of rotation of the motor, either two of the transistors or another two of the transistors are conducting. Upon motor shutoff or switchover, because of the energy stored in the inductance of the motor, negative potential peaks that fall below the ground potential of the operating voltage appear at the motor terminals and can cause so-called substrate currents (on the substrate of the integrated circuit), which can cause the failure of entire functional portions of the integrated circuit.
Until now, external guard diodes have been used which have a diffusion voltage that is below the diffusion voltage of the parasitic substrate diodes of the integrated circuit. Expensive, nonintegrated Schottky diodes have typically been used for that purpose.
A circuit for triggering a DC motor through an H-bridge circuit is known from German Published, Prosecuted Patent Application DE-AS 1 130 042. In order to protect against harmful induction peak voltages, that circuit uses Zener diodes which are connected parallel in the depletion direction to the power transistors connected to the positive pole of the operating voltage. If the operating voltage fluctuates, as happens, for instance, in on-board motor vehicle electrical systems, substrate currents on integrated circuits cannot be avoided with such a structure.
A circuit configuration for triggering a DC motor through an H-bridge circuit is known from German Published, Non-Prosecuted Patent Application DE 40 38 199 A1. In order to limit transient voltage and/or current peaks when the motor is turned on, shut off or switched over, that device contemplates driving a transistor in the just-opened current path in clocked fashion, or short-circuiting the motor through both transistors connected to the same pole of the operating voltage. Once again, that does not make it possible to limit negative voltages to a value that keeps the substrate diodes nonconducting.